Purpose/Objective: We have investigated the use of two new dosimetry systems for small field dosimetry. The first system is based on Al2O3:C radioluminescence (RL) (Radiat Meas, 46 (10), 109098, 2011). The main part of the RL dosimetry system is a small (2x0.5x0.5 mm3) Al2O3:C crystal (Landauer Inc, USA). The RL signal generated in the crystal by ionizing radiation can be read remotely via thin optical fiber cables. The system was originally developed for in vivo dose verification during external beam radiotherapy and brachytherapy (Radiother Oncol, 100 (3), 45662, 2011). However, due to the small dimensions of the Al2O3 crystal, the system may have applications in small field dosimetry. The second system is the RADPOS system (Med Phys, 36, 167279, 2009), a novel 4D dosimetry system available from BEST Medical Canada. RADPOS probe consists of 2 sensors: a small antenna as an electromagnetic positioning sensor and a μMOSFET for dose measurement.

Materials and Methods: Relative output factors (ROF) for Cyberknife cones ranging from 5 to 60 mm were measured using RL and RADPOS systems. For comparison, measurements were also carried out using a mobileMOSFET system (BEST Medical Canada) and GafChromic films EBT1 and EBT2 (ISP, USA). The MOSFET detectors in both mobileMOSFET and RADPOS systems were standard sensitivity μMOSFETs (TN502RDM), with a standard bias applied during irradiation. The measurements were performed in a solid water phantom at the depth of 1.5 cm and SSD = 78.5 cm. Detector readings for each cone were normalized to those for 60 mm cone. For MOSFET detectors in both mobileMOSFET and RADPOS systems, the corrections proposed by Francescon et al. (J Appl Clin Med Phys, 10 (1), 14752, 2009) were applied. Since FWHM of our Cyberknife source is 2.4mm, the μMOSFET correction factors based on interpolation of Francescon et al. values are 0.956 and 0.992 for 5 and 7.5 mm cones, respectively.

Results: The μMOSFET/RADPOS measurements (corrections applied) yielded ROF of 0.650+/ 1.8% and 0.811+/ 0.8% for 5 and 7.5 mm cones, respectively, and were in excellent agreement with GafChromic film values (averaged for EBT1 and EBT2) of 0.645+/2% and 0.807+/2%. Assuming EBT film as a 'golden standard' (no correction required), the RL system overestimated the ROF for 5 mm cone by 5.5% and 3.9% for 5 and 7.5 mm cones, respectively. This is in agreement with Francescon et al. hypothesis that small solid state detectors require a correction factor lower than 1 applied to their readings to correct for excessive scatter due to relative high atomic number (10.2 for Al2O3) compared to water. For cone sizes 1060 mm all detectors gave results in excellent agreement, with differences well within the measurement uncertainty for each detector.